Our research has revealed the presence of long- and short-lived lymphocytes in the avian thymus, bursa of Fabricius, spleen, and bone marrow. The latter lymphomyeloid tissue possesses the longest lived population of lymphocytes. Whether the long-lived population in the bone marrow is bursa or thymic derived is not known. Therefore, the initial objective of this proposal will be to functionally classify the origin of bone marrow lymphocyte and to assess the lifespan of bone marrow lymphocytes in bursaless chickens. Application of steroids and cyclophosphamide (Cy) in ovo will eliminate B-cells in a large percentage of our birds. While the injection of neonatal chickens with Cy will not eliminate peripheral B-cells, Cy does expand a population of cells in the bursa that morphologically exhibit secretory capabilities and appear to secrete actively. Since Cy and other immunosuppressants are useful in transplantation and in the treatment of human cancer, we need to understand Cy-mediated changes in vertebrates. Therefore, we plan to study the biological activity of the product from the secretory cell.